How come a promotion is a scam?
Promotion is just a marketing strategy to most businesses including the gambling casino, their main target is to attract lots of customers to play on their casino. The offer may seem to be smaller to you but for the other gamblers, it is enough already, at least they got some bonuses with that promotion. It is up to us too to find whether we like to play on that casino or not since nobody is forcing you to play on that site.
He didn't said all promotions are scam. It's possible that some promotions could become a scam because sometimes casinos hide the terms and conditions of the promotion and could suddenly add an extra rule once a player starts winning too much than expected. I agree that we're not forced join a promotion but it's hard to avoid when it seems like free money.
Most of the time promotions won't be getting termed to be scams, in promotional marketing scams were commonly experienced with bounties. As every process is associated with internet even legit way of promotion go as a scam. Recently one happened with icoforums.net, which was growing good with better way of promotion and served the participants good. In the mean manager took away the funds and ran away.
It is quite common with other ICO projects as well as the developer acts as the authority and could be a scammer. As described by you, the manager is also a scammer so better we should go for projects which have both good promotions and famous for its legit functions. Gambling sites have also promotions and should do it because promotion is one of the four Ps of a successful business that should not be ignored.